Remote microwave plasmas are routinely used in semiconductor processing for the generation of radical species for etching and deposition. Remote microwave sources generate intense electric fields within a cavity to form a plasma which then diffuses into the processing chamber as an afterglow plasma. While conventional cavity devices employ a simple construction, they suffer from several drawbacks. Since the plasma is itself a lossy dielectric, its presence in the cavity modifies the initial electric field distribution resulting in a substantial change in the impedance of the cavity. Often the associated tuning stubs have to be adjusted after the plasma has been generated to achieve a proper impedance match and optimum power coupling. Further, the power dissipation occurs within the entire cavity and proper cooling of the cavity is essential, since at high power levels the discharge tube may be damaged by the high thermal load. Finally, transport of the plasma outside the cavity occurs solely by diffusion. Thus the afterglow plasma in the chamber may be quite weak. Surface wave discharges offer several advantages over cavity sustained discharges. Unlike conventional cavity sustained discharges, the plasma is generated in a short gap, which is only a small portion of the entire cavity such that the cavity operates much cooler, except in the vicinity of the short gap. Only a small fraction of the total power is dissipated in the short gap; the rest is radiated. Thus, the impedance of the cavity, unlike conventional cavities, is not sensitive to the plasma conditions. Further, the surface wave propagating along the discharge tube produces long plasma columns providing a denser plasma within the chamber. Compared to conventional cavities which require a 3 or 4 stub tuner, impedance matching for the surface wave discharge is achieved by moveable shorts in the coaxial and rectangular waveguide sections. Thus tuning is simple and relatively independent of plasma conditions because of the nature of the cavity.
The conventional surface wave plasma generators however also have substantial disadvantages. Higher power surface waves are characterized by stronger electric fields. The maximum power handling capacity is limited by electric field induced breakdown in the gap. If the field becomes too intense, arcing can occur in the gap causing failure. Even if arcing does not occur, the short gap may become extremely hot. If the gap is too long, surface waves cannot be launched into the discharge tube. Additionally, for certain geometries and process conditions it is difficult to strike a plasma without the aid of an external trigger, such as an electric spark or a Tesla coil. Both these external triggers are a source of electrical noise in the reactor, and are therefore undesirable. Finally a minimum plasma column length is required to completely absorb the microwave power.
Thus the need has arisen for improved methods and apparatus for generating microwave plasmas. Such improved apparatus and methods should generate a plasma without the need for extensive tuning and cooling mechanisms. Further, the improved apparatus and methods should generate sufficiently strong plasmas for use in such applications such as semiconductor fabrication systems.